1. Field of the Invention
The present invention relates to optical technique that can be applied to a semi-transparent liquid crystal display.
2. Description of the Related Art
Liquid crystal displays have characteristics of thin-shaped, lightweight and low power consumption. Thus, in recent years, their application to mobile devices and stationary equipments such as television receivers increases rapidly.
Some liquid crystal displays, for example, those mounted on mobile devices are desired to attain high visibility not only in interior lighting environments and dark places but also under high-luminance light source such as sun. Semi-transparent liquid crystal displays are those which fit such demands and are mounted in many mobile devices.
In such a semi-transparent liquid crystal display, each pixel contains a transmissive display area and a reflective display area. In the transmissive display area, a transparent conductive layer is used as the back electrode whereas in the reflective display area, a layer of metal or alloy is used as a part of the back electrode. Also, in the transmissive display area, light which is transmitted once through the coloring layer of a color filter is utilized for display whereas in the reflective display area, light which is transmitted twice through the coloring layer of a color filter is utilized for display. Therefore, the reflective display area is provided with a coloring layer having a higher transmittance than that of the transmissive display area. Because the semi-transparent liquid crystal display adopts such a structure, a multicolor image can be displayed by both the transmissive and reflective systems.
In the semi-transparent liquid crystal display, wavelength plates such as a quarter-wave plate are used. For example, there is the case where a retardation film as a quarter-wave plate is disposed between the liquid crystal cell and the front side polarizing plate and a retardation film as a quarter-wave plate is further disposed between the liquid crystal cell and the backside polarizing plate. However, when two quarter-wave plates are used, there is the case where an avoidable variation in the characteristics of these quarter-wave plates leads to a drop in the contrast ratio to be attained by transmissive display.
In relation to such a problem, JP-A 2004-4494 describes that a patterned retardation layer is disposed inside of a liquid crystal cell instead of placing a retardation film on the liquid crystal cell. Specifically, a retardation layer made of a polymer liquid crystal and an optional organic insulation layer are disposed only in the reflective display area to reduce the cell gap in the reflective display area with respect to that in the transmissive display area. This allows a structure that does not include a quarter-wave plate in the transmissive display area and therefore, transmissive display having a high contrast ratio is attained.
However, the liquid crystal display described in JP-A 2004-4494 also has a problem concerning reflective display. This problem resides in the point that although red, green and blue pixels are different in the wavelength range of the display color, the retardation layer of the reflective section has the same characteristics for each color and it is therefore difficult to adopt an optimum design for all pixels differing in display color.
Specifically, when a quarter-wave plate by which quarter wavelength (λ/4) is obtained at the center wavelength of the green wavelength range, for example, about 550 nm is used, a retardation larger than λ/4 is obtained in the blue wavelength range having a center wavelength of about 450 nm, even if it is supposed that the refractive index anisotropy of this quarter-wave plate, that is, birefringence Δn is almost the same throughout the entire visible range. Then, a retardation smaller than λ/4 is obtained in the red wavelength range having a center wavelength of about 630 nm. Actually, in many optical materials, the birefringence is larger in the shorter wavelength side, that is, in the blue wavelength range and is smaller in the longer wavelength side, that is, in the red wavelength range, so that this problem is sometimes more serious.
Also, JP-A 2005-24919 describes that a retardation layer is disposed inside of a liquid crystal cell instead of placing a retardation film on the liquid crystal cell. A retardation suitable to each color pattern is obtained by changing the thickness of the retardation layer correspondingly to red, green and blue patterns. This is attained by forming a color filter layer which is to be the base of the retardation layer, such that red, green and blue color filter layers, have different thicknesses, which allows the retardation layer subsequently applied to have different thickness is correspondingly the colors. This makes it possible to obtain a retardation layer having optimized retardations which vary depending on colors.
However, in the method described in JP-A 2005-24919, it is necessary to vary the thickness of the color filter layer correspondingly to the color of patterned layers, and also, the color filter is limited in its design. This is caused by the fact that in each pixel of the semi-transparent liquid crystal display, the coloring layer of the reflective display area is required to have a higher transmittance than the coloring layer of the transmissive display area. This will be described with reference to the structure of the color filters for each of the reflective display area and transmissive display area.
Such a difference in transmittance can be produced, for example, by using, in the coloring layer of the transmissive display area, a material different from that used in the coloring layer of the reflective display area. In this case, it is necessary to form red, green and blue coloring layers for the transmissive display area and red, green and blue coloring layers for the reflective display area. Namely, in the case of adopting this method, the process of forming the color filter layer is complicated. Also, a larger number of materials are required.
Alternatively, it is possible that the same material is used for the coloring layer of the transmissive display area and coloring layer of the reflective display area in each pixel and the coloring layer of the transmissive display area is made to be thicker than the coloring layer of the reflective display area, to produce the aforementioned difference in transmittance. However, it is highly difficult to strictly control both the thickness of the coloring layer in the transmissive display area and thickness of the coloring layer in the reflective display area as compared with the case of strictly controlling the thickness of the coloring layer having a uniform thickness.
The aforementioned difference in transmittance can be produced by adopting in each pixel the same structure in the coloring layer of the reflective display area and in the coloring layer of the transmissive display area except that through-holes are formed in the coloring layer of the reflective display area. According to this method, the color filter layer can be formed more easily.
If it is intended to use the method described in JP-A 2005-024919, the structure in which through-holes are formed in the coloring layer of the reflective display area cannot be adopted. The retardation layer is formed in the reflective display area on the premise that the wavelength of transmitted light differs for each color and the retardation value is designed according to this. Because white light passes as-is at the through-hole section where no coloring layer exists, the contrast in reflective display is rather lowered since the retardation value is adjusted according to each color.